1. Field of the Invention
The present invention relates to a magnetoresistive device comprising a magnetoresistive element for reading magnetic signals and a method of manufacturing such a magnetoresistive device, and to a thin-film magnetic head, a head gimbal assembly and a hard disk drive each including a magnetoresistive device.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as a real recording density of hard disk drives has increased. Such thin-film magnetic heads include composite thin-film magnetic heads that have been widely used. A composite head is made of a layered structure including a write (recording) head having an induction-type electromagnetic transducer for writing and a read (reproducing) head having a magnetoresistive (MR) element for reading.
MR elements include: anisotropic magnetoresistive (AMR) elements utilizing an anisotropic magnetoresistive effect; giant magnetoresistive (GMR) elements utilizing a giant magnetoresistive effect; and tunnel magnetoresistive (TMR) elements utilizing a tunnel magnetoresistive effect.
High sensitivity and high output capability are required as characteristics of a read head. GMR heads incorporating spin-valve GMR elements have been mass-produced as read heads that satisfy such requirements. In this type of GMR head the GMR element is inserted between a bottom shield layer and a top shield layer. Each of the top and bottom shield layers is made of a magnetic metal material such as NiFe or FeAlSi.
Conventional GMR heads have a structure in which a current used for detecting magnetic signals (that is hereinafter called a sense current) is fed in the direction parallel to a plane of each film making up the GMR element. Such a structure is called a current-in-plane (CIP) structure. In the GMR head having the CIP structure, the GMR element is insulated from each of the top and bottom shield layers by an insulating film. As a result, there arises a problem that, if the space between the top and bottom shield layers is reduced to enhance the linear recording density of the GMR head having the CIP structure, the above-mentioned insulating film is made thin and it is difficult to maintain the insulation between the GMR element and each of the shield layers.
The heads proposed to solve such a problem are GMR heads having a structure in which a sense current is fed in the direction perpendicular to a plane of each film making up the GMR element. Such a structure is called a current-perpendicular-to-plane (CPP) structure. It is not necessary to insulate the GMR element from each of the shield layers for the GMR head having the CPP structure. Therefore, the GMR head having the CPP structure is free from the above-mentioned problem. A TMR head incorporating a TMR element has the CPP structure, too.
The characteristics of an MR element are affected by the crystallinity and orientability of the films making up the MR element. Therefore, an underlying layer is often provided under the MR element for the purpose of improving the crystallinity and orientability of the films making up the MR element. When such an underlying layer is provided in a head having the CPP structure, the films making up the MR element are formed on the underlying layer provided on the bottom shield layer made of a magnetic metal material.
GMR heads having the CPP structure are disclosed in the Published Japanese Translation of PCT International Publication for Patent Application Heisei 11-509956 (1999) (hereinafter referred to as “JP-A-H11-509956”) and the Published Unexamined Japanese Patent Application 2003-60262 (hereinafter referred to as “JP-A-2003-60262”). In the GMR head disclosed in JP-A-H11-509956, a conductor layer is formed directly on the bottom shield layer or formed on an insulating gap layer that is provided on the bottom shield layer, and a spin-valve GMR element is formed on the conductor layer. The conductor layer is made of a material including any of Rh, Al, Au, Ta and Ag, or an alloy of these elements.
In the GMR head disclosed in JP-A-2003-60262, a spin-valve GMR element is formed on a bottom metal layer that is provided on the bottom shield layer. The bottom metal layer is made up of a Ta layer and an NiFe layer formed on the Ta layer, for example.
As disclosed in JP-A-H11-509956, when the MR element is formed on the single conductor layer made of a material such as Rh, Al, Au, Ta or Ag that is provided on the bottom shield layer, it is difficult to form films having excellent crystallinity and orientability as the films making up the MR element. The reason would be that the surface of the above-mentioned conductor layer has a poor wettability.
Furthermore, in the head having the CPP structure, as disclosed in JP-A-2003-60262, when the MR element is formed on the bottom metal layer made up of the two layers of Ta and NiFe provided on the bottom shield layer, it is possible to form films having excellent crystallinity and orientability as the films making up the MR element. In this case, however, it is difficult to improve the characteristics of the MR element such as an MR ratio (a ratio of magnetoresistive change to the resistance).